Presently in the United States, the television broadcasting picture signals are limited to a bandwidth having an upper band frequency of 4.2 megahertz as set by the National Television Standards Committee (NTSC). It is commonly known that the fine details of a video picture are generated by the high frequency components of the video signal, whereas the lower frequency components generate the larger areas of the picture. However, the aforementioned conventional video bandwidth places a limitation on the resolution of the displayed images by restricting the transmission of the high frequency video components above 4.2 megahertz.
In a number of countries outside the United States, the broadcast television signals have frequency bandwidths which exceed five megahertz. The resulting images are generally considered to be superior to those broadcast in the United States because of the increased number of picture elements which are generated by the higher frequency components of the broadcast signals.
In order to increase picture resolution while staying within the video transmission frequency constraints, various enhancement techniques have been utilized. For example, a conventional technique is to generate video pulses which have transient undershoot and overshoots in the time domain. Presence of these undershoots and overshoots during a light-to-dark pulse transition or vice versa, gives the appearance of greater contrast between the light and dark picture elements and this gives the appearance of increased clarity. It is not uncommon, however, for unwanted effects such as signal "ringing" to accompany the generation of these transient overshoots. This ringing detracts from the picture quality by generating a white border at the edges of the displayed images and is often referred to as "white edge ringing".
Other methods for enhancing video images have also been disclosed. In U.S. Pat. No. 3,789,133 by Kline, there is disclosed a television camera aperture correction system in which the high frequency components are separated from the video signal. These high frequency components are amplified, the unwanted noise is removed, and the filtered high frequency signal components are added back into the video signal.
A video crispener is disclosed in U.S. Pat. No. 4,030,121 by Faroudja, in which the transition times of video signal pulses are shortened to provide sharper luminance and chroma information. This is accomplished by multiplying a function of the full wave rectified first differential of the input signal by a function of the second differential of either the input signal or another signal, and then adding the wave form resulting from their product to the properly delayed input signal.
Other video crispeners include U.S. Pat. No. 2,740,071 by Gcldmark et al; U.S. Pat. No. 2,851,522 by Hollywood; U.S. Pat. No. 2,863,999 by French; and U.S. Pat. No. 2,678,964 by Loughlin.
A contour correcting circuit for decreasing the transition time of a video pulse and for eliminating preshoot and overshoot in order to enhance picture quality is disclosed in U.S. Pat. No. 4,541,014 by Yagi.
In U.S. Pat. No. 4,437,122 by Walsh et al, there is disclosed a system for enhancing the resolution of video display pixels for hard copy printouts wherein a decoder compares the pixels on the same raster line, as well as in preceding and succeeding raster lines that surround each pixel, to generate print head driving signals.
In addition, Ranalli, U.S. Pat. No. 4,382,254 discloses a multilevel video display control circuit for controlling the brightness of the pixels on a display by summing together currents developed in response to different command signals in order to reduce fringing effects.
In U.S. Pat. No. 3,946,152 by Illetschko et al, there is disclosed a correction system for video signals.